1. Benoit Lavraud CESR/CNRS, Toulouse, France Uppsala, May 2008 The altered solar wind – magnetosphere interaction at low Mach numbers: Magnetosheath and magnetopause

2. OUTLINE Introduction and motivation Magnetosheath β properties Asymmetric flows in the magnetosheath Asymmetric magnetopause shape Kelvin-Helmholtz instability Other expected effects Occurrence distribution of solar wind MA Conclusions Note: we primarily use global MHD simulations here

3. INTRODUCTION : Mach numbers, shocks and plasma β - Mach numbers: Alfvén M A = V SW / V A with V A ~|B|/√N Magnetosonic M MS = V SW / √(V A 2 + V S 2 ) So that M A > M MS - Plasma β: - Rankine-Hugoniot shock jump conditions: B downstream = f(B upstream ) N downstream = f(N upstream ) etc.

4. MOTIVATION : An “unknown” or “over-looked” magnetosphere - Pivotal role of magnetosheath - Implications for CME-driven storms Lobes Plasma sheet Magnetopause Solar wind Magnetosheath High Mach number = High-β magnetosheath Cusp Low Mach number = Low-β magnetosheath

5. Magnetosheath β as a function of SW Mach number  Low-β magnetosheath prevails during low Mach numbers: Magnetic forces become important - Rankine-Hugoniot shock Jump conditions - M MS = V SW / √(V A 2 + V S 2 ) - M A = V SW / V A - M A > M MS Varying IMF Perpendicular shock case

6. Magnetosheath flow dependence on Mach number  Strong flow acceleration : increasing for decreasing MA Global MHD simulations (BATS-R-US) for high and low Mach numbers Equatorial planes

7. Magnetosheath flow acceleration and asymmetry  Asymmetric flow acceleration, along the flanks only: a magnetic “slingshot” effect? Global MHD simulation (BATS-R-US) for low Mach number (M A = 2) Equatorial planeX = -5 R E

8. Mechanism of magnetosheath flow acceleration  We can estimate the contribution of each force: J x B acceleration dominates at low Mach numbers - Steady state momentum equation: - Magnetic forces - Integration of forces: Selection of streamline ∂s∂s MHD simulation for low M A Y (R E ) Z (R E ) Note: Not a simple analogy to a “slingshot”, magnetic pressure gradient as important as tension force (~10% 45% 45%)

9. Observation of magnetosheath flow jets  Flows not associated with reconnection and 60% > SW - Solar wind observations: IMF large and north SW density low - Cluster observations: Flows B field outside MP Up to 1040 km/s while SW is only 650 km/s Electrons Ions Sheath sheath shock dusk Cluster SW speed Magnetopause See also: Rosenqvist et al. [2007]

10. Flow asymmetry: role of IMF direction  The enhanced flow location follows the IMF orientation Flow magnitude and sample field lines from MHD simulations (X = -5 R E )

11. Magnetopause asymmetry: role of magnetic forces  The magnetopause is squeezed owing to enhanced magnetic forces in the magnetosheath Current magnitude and sample field lines from MHD simulations (X = -5 R E )

12. Magnetopause asymmetry: role of IMF direction  The magnetopause squeezing follows the IMF orientation Current magnitude and sample field lines from MHD simulation (X = -5 R E )

13. Occurrence of the Kelvin-Helmholtz instability  May expect KH instability to grow faster with larger flows - Used their list of such KH vortices to inspect their dependence on M A - Rolled-up KH vortices may be identified in data Hasegawa et al. [2006] N & V high N & V low Vx Vx higher Sheath Sphere

14. Occurrence of giant spiral auroral features  KH instability, large flow and spiral aurora may be related - Giant spiral auroral features have been identified during great storms (Dst < -250 nT) ( courtesy of J. Kozyra ) + case by Rosenqvist et al. [2007]  7 out of 8 occurred for M A < 5 See: Rosenqvist et al. [2007]

15. SOME OTHER LOW M A EFFECTS Changes in dayside reconnection rate Cross-polar cap potential saturation Sawtooth oscillations Plasma depletion layer (disappears at high M A ) Heating at bow shock (Ti/Te and tracing) Drifts and losses to the magnetopause (radiation belts and ring current) Alfvén wings in sub-Alfvénic flow Bow shock acceleration and reflection else …?

16. Occurrence distribution of solar wind Mach numbers  CMEs, and particularly the subset of magnetic clouds, have low Mach numbers - Binning of OMNI dataset - Lists from: CMEs: Cane and Richardson [2003] MCs: Lepping et al. [2006] HSS: Borovsky and Denton [2006] See also: Gosling et al. [1987] Borovsky and Denton [2006]

17. CONCLUSIONS SW – magnetosphere interaction is significantly altered during low Mach number All these effects are thus important during CME-driven storms They must occur at other magnetospheres (Mercury: low M A and no ionosphere Moons, e.g., like Io in sub-Alfvenic flows)

18. Acknowledgments Joseph E. Borovsky (Los Alamos, USA) Aaron J. Ridley (Univ. Michigan, USA) Janet Kozyra (Univ. Michigan, USA) Maria M. Kuznetsova and CCMC (NASA GSFC, USA) and Cluster teams